Militant group Hizb-e-Islami claimed Tuesday's suicide bombing in Kabul that killed 12 civilians. The group had been more discriminating in targets, and more engaged in peace talks.

Following a suicide bombing in Kabul on Tuesday that left 12 people dead, including eight South Africans and a Kyrgyz, the insurgent group Hizb-e-Islami stepped forward to claim responsibility.

As one of the most moderate Islamic militant organizations in Afghanistan, the group’s involvement came as a shock to many of its supporters. Over the course of the past decade, the group has violently opposed international forces, but called on fighters not to hurt civilians or damage the country’s infrastructure.

The militant group has also been among the most receptive to peace talks with the US, NATO, and the Afghan government. And a separate, nonviolent faction of the group had already integrated into mainstream politics, with Hamid Karzai picking members for cabinet posts and adviser roles. As one of the few political parties with decades of experience and members across ethnic groups, the nonviolent faction has been seen as a growing player in the country's political scrum as Mr. Karzai faces a term limit and the bulk of international troops leave in 2014.

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After Tuesday’s bombing, however, many people tied to Hezb-e-Islami, especially those in the nonviolent faction, worry that the militant faction may have lost the bargaining power they once held at the negotiating table and done damage to the nonviolent faction's ascendant political brand.

“After this attack there won’t be a big chance for Hizb-e-Islami to negotiate with the Afghan government,” says Abdul Jabar Shulgari, a former member of Hizb-e-Islami nonviolent wing. “This bombing now has shown the Afghan nation that Hizb-e-Islami is not as moderate as they thought it was. This attack brings many questions to the mind of the people and raises many questions about the credibility of Hizb-e-Islami.”

Suicide attacks like Tuesday’s that target civilians, even foreign civilians, are so out of character for the group that many Hizb-e-Islami supporters have denied the possibility of its involvement even after the organization's leadership, including the top spokesman, released an official statement taking responsibility.

“It looks almost impossible for me. I am still suspicious about whether this act was actually conducted by Hizb-e-Islami. Anyone can change their voice and call the media to claim responsibility for an attack,” says Mangal Sherzad, who represented Hizb-e-Islami to Germany during the Soviet war and who remains a member. “If they did it, I’m not sure why they did it at this particular time when they are near a deal with the Afghan government.”

For many like Mr. Sherzad, Hizb-e-Islami’s sudden departure from its traditional strategy to avoid suicide attacks and civilian casualties has caused much confusion and frustration.

After Tuesday’s bombing, many supporters and Afghans now wonder if the group can maintain what legitimacy it once had on the political stage and if the event will prove fatal to what popular support the organization once had.

“Hizb-e-Islami is viewed as a liberal opponent of the government, so in the long-term it will negatively affect their standing among the Afghans if they intensify their fighting and carry out more attacks, but they still want to show themselves as a strong player in the current conflict,” says Muhammad Hassan Haqyar, an independent political analyst in Kabul.

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Hizb-e-Islami is broken into two factions, one led by Gulbuddin Hekmatyar that violently opposes the presence of foreign forces, and the other side largely loyal to the Afghan government. Still many in the nonviolent faction of the group remain sympathetic to Mr. Hekmatyar and his supporters.

The question remains whether Hekmatyar and his supporters still want to negotiate. Following the signing of the strategic partnership agreement between the US and Afghanistan in May, many insiders say that the group has shied away from talks after the deal paved the way for a long-term American presence here until at least 2024. Even before the deal, some close observers of the talks suggested that those in the group most amenable to peace had already opted for the mainstream faction, leaving the militant faction with an inflexible core of loyalists around Hekmatyar who were unlikely to cut a deal.

“If the Americans are honest in negotiations, we are ready for negotiations, but they always tell lies. As long as there is just one American here we will continue to fight,” says Haroon Zarghoon, a spokesman for Hizb-e-Islami's militant faction.